What would I be getting if I bought the player's handbook digitally?

Just curious because I don't have a lot of money, and I already plan to buy Monsters of the Multiverse, and a subscription to share content, so I want to know if it's worth it.

I've never looked at the freely available portion of the rules but here's a summary after a search that sounds right on some of the big things I have gathered in discussions here on DDB. It's entirely possible something more may have been added to the free rules since this summary, but in general you're getting substantially more player options. Whether or not this is important to you only you know. From an actual key game rules standpoint I don't have any familiarity with anything that might be missing from the free set.

• Classes/Subclasses: The paid PHB contains 48 total subclasses (4 per class), while the free version restricts players to one core subclass per class (for example only Thief Rogue, Life Cleric).
• Species & Backgrounds: The free rules limit options to 4 species and 4 backgrounds, missing most of the new origin options found in the full PHB.
• Feats & Spells: The free rules include a limited selection of 12 feats and about 320 spells, whereas the full PHB boasts all 75+ feats and a larger, revised spell list.

If you don't already own the Monster Manual I am a bit surprised you would be considering the Monsters of the Multiverse book, but hopefully you have already researched and understand the difference between the two.